【摘要】：吸收式制冷機和吸收式熱泵不僅能夠有效利用余熱、提高能源利用率,還能夠節(jié)約電能、降低建筑能耗,因而具有巨大的發(fā)展空間。吸收器是吸收式制冷系統(tǒng)的重要部件,深入研究強化吸收式制冷系統(tǒng)的吸收器結(jié)構(gòu)及其內(nèi)部傳熱傳質(zhì)機理,不僅具有重要的學術(shù)價值而且還有廣闊的應用前景。水平管束降膜吸收器具有傳熱溫差小、傳熱效率高、設備成本低和流動阻力小等特點,是吸收器的主要和重要形式。本文圍繞溴化鋰溶液在水平管束降膜吸收器中的流動特性展開,系統(tǒng)地分析了溶液在水平管外表面的瞬態(tài)鋪展特性和達到穩(wěn)定后液膜厚度沿周向角的變化趨勢,為水平管束降膜吸收機理的研究和吸收器結(jié)構(gòu)強化傳熱的設計提供重要依據(jù)。本文針對水平管束降膜流動的特點,建立了溴化鋰溶液和水蒸汽在水平管外降膜流動的物理及數(shù)學模型;基于數(shù)值模擬軟件COMSOL中的層流兩相流Level set物理場,采用有限元法,數(shù)值模擬了不同吸收器結(jié)構(gòu)和流體流動參數(shù)下溴化鋰溶液在水平管外表面的降膜流動特性。研究了溶液在水平管外表面的瞬態(tài)鋪展特性,詳細分析了特定時刻溴化鋰溶液在水平管外表面的布液形態(tài)。結(jié)果表明,a)在壁面靜態(tài)接觸角為零的理想水平管外壁,溴化鋰溶液能夠鋪展一層完整的液膜,當水平管外壁的溶液噴淋密度減小和壁面潤濕性變差即壁面靜態(tài)接觸角增大時,溶液在水平管外壁表面出現(xiàn)局部干斑,其鋪展形態(tài)由膜狀流轉(zhuǎn)化為滴狀流。b)溴化鋰溶液在水平管外壁的覆蓋率隨噴淋密度和壁面潤濕性的增加或水平管徑的減小而增大;溶液鋪展到特定周向角處所需的時間隨噴淋密度或液膜雷諾數(shù)的減小以及壁面靜態(tài)接觸角的增大而延長。c)布液高度較大和/或噴淋密度減小到一定程度時,溴化鋰溶液在水平管外壁的覆蓋率隨著液膜收縮和新溶液的注入而波動,呈周期性變化;在降膜流動過程中,水平管下半圓周的液膜波動幅度大于上半周的液膜波動幅度,且液膜在水平管外壁的波動幅度隨著壁面靜態(tài)接觸角減小而增強。研究了液膜在水平管外表面的穩(wěn)態(tài)特征,獲得了達到穩(wěn)定后周向角20°～160°范圍內(nèi)(間隔10°)液膜厚度和速度的大小,詳細分析了液膜厚度和速度沿周向角的分布趨勢,探究了吸收器結(jié)構(gòu)和流體流動參數(shù)對液膜厚度和速度的影響。結(jié)果表明,a)水平管外表面的液膜厚度沿水平管圓周呈先減小后增大的趨勢,水平管下半周的液膜平均厚度小于上半周的液膜平均厚度,液膜厚度最小值出現(xiàn)在周向角120°附近;基于模擬結(jié)果,并對比前人的模擬和實驗結(jié)果,對Nusselt液膜厚度沿周向角的分布公式進行了修正。b)液膜厚度隨噴淋密度的增加而增大,隨水平管徑和/或布液高度的增大而減小;當水蒸汽流動方向與液膜鋪展方向一致時,達到穩(wěn)定后液膜厚度隨著蒸汽流速的增大而減小,液膜表面速度隨著蒸汽流速的增大而有所增大,且上半周的液膜表面速度隨著蒸汽流速的變化幅度大于下半周;當水蒸汽流動方向與液膜流動方向相反時,蒸汽流動僅對下半周液膜厚度和液膜表面流速有影響,使下半周液膜厚度隨著蒸汽逆向流速的增大而減薄,下半周的液膜表面速度隨著蒸汽逆向流速的增大亦有所增大。c)沿管壁的法向方向液膜內(nèi)部流動速度逐漸增大,汽液交界處的液膜表面速度達到最大值,達到穩(wěn)定狀態(tài)后液膜表面速度沿周向角的變化趨勢與液膜厚度沿圓周方向的變化趨勢相反,液膜表面速度沿周向角呈先增大后減小的變化趨勢,液膜表面速度的最大值和液膜厚度最小值都出現(xiàn)在周向角120°附近。在實際的降膜吸收過程中,當噴淋密度較小和/或壁面靜態(tài)接觸角較大時,溶液在水平管外壁的覆蓋率較低,降膜吸收的有效面積較小,溶液在干斑區(qū)域所需的鋪展時間延長;當噴淋密度過大和/或水平管徑過小時,鋪展過程中溶液在液膜前端的堆積量增大,達到穩(wěn)定后水平管外壁的液膜較厚,這些都對降膜吸收不利。因此,需要根據(jù)實際應用選擇合理的吸收器結(jié)構(gòu)參數(shù)并優(yōu)化相對應的噴淋密度,也可通過增強水平管壁面潤濕性、調(diào)整布液高度等方法改善降膜吸收的性能。
[Abstract]:Absorption chiller and absorption heat pump can not only utilize waste heat effectively, improve energy utilization rate, but also save electric energy and reduce building energy consumption. Absorber is an important component of absorption refrigeration system. The horizontal tube bundle falling film absorber is the main and important form of the absorber because of its small temperature difference, high heat transfer efficiency, low equipment cost and small flow resistance. The transient spreading characteristics of solution on the outer surface of horizontal tube and the variation trend of liquid film thickness along circumferential angle after stabilization are systematically analyzed, which provides important basis for the study of absorption mechanism of falling film of horizontal tube bundle and the design of heat transfer enhancement of absorber structure. Based on the Level set physical field of laminar two-phase flow in COMSOL, the falling film flow characteristics of LiBr solution on the surface of horizontal tube with different absorber structures and fluid flow parameters were simulated by finite element method. The results show that (a) LiBr solution can spread a complete liquid film on the outer wall of an ideal horizontal tube with zero static contact angle on the wall. When the spray density of the solution on the outer wall of the horizontal tube decreases and the wettability of the wall becomes worse, the liquid distribution on the outer surface of the horizontal tube becomes worse. When the static contact angle of the wall increases, a local dry spot appears on the outer surface of the horizontal pipe, and the spreading pattern changes from film flow to droplet flow. When the spray density or Reynolds number of the liquid film decreases and the static contact angle of the wall increases, the liquid distribution height and/or the spray density decrease to a certain extent, the coverage of the LiBr solution fluctuates periodically with the shrinkage of the liquid film and the injection of the new solution. The fluctuation amplitude of the liquid film in the lower half of the circle is larger than that in the upper half of the circle, and the fluctuation amplitude of the liquid film in the outer wall of the horizontal pipe increases with the decrease of the static contact angle of the wall. The distribution trend of liquid film thickness and velocity along circumferential angle is analyzed in detail, and the influence of absorber structure and fluid flow parameters on liquid film thickness and velocity is investigated. Based on the simulation results, the distribution formula of Nusselt liquid film thickness along the circumferential angle is revised by comparing the simulation and experimental results. b) The liquid film thickness increases with the increase of spray density and decreases with the increase of horizontal pipe diameter and / or liquid distribution height. When the water vapor flow direction is consistent with the spreading direction of the liquid film, the thickness of the liquid film decreases with the increase of the steam flow velocity, and the surface velocity of the liquid film increases with the increase of the steam flow velocity. When the flow direction is opposite, the vapor flow only affects the thickness of the liquid film and the surface velocity of the liquid film in the second half of the cycle. The thickness of the liquid film in the second half of the cycle decreases with the increase of the reverse flow velocity of the steam. The surface velocity of the liquid film in the second half of the cycle also increases with the increase of the reverse flow velocity of the steam. The liquid film surface velocity at the vapor-liquid junction reaches the maximum value. After reaching the stable state, the change trend of the liquid film surface velocity along the circumferential angle is opposite to that of the liquid film thickness along the circumferential direction. The liquid film surface velocity increases first and then decreases along the circumferential angle. The maximum value of the liquid film surface velocity and the minimum value of the liquid film thickness are both. In the actual falling film absorption process, when the spray density is small and/or the static contact angle of the wall is large, the coverage of the solution on the outer wall of the horizontal tube is low, the effective area of falling film absorption is small, and the spreading time of the solution in the dry spot area is prolonged. During the spreading process, the accumulation of solution in the front of the liquid film increases, and the liquid film on the outer wall of the horizontal pipe becomes thicker after stabilization, which is disadvantageous to the absorption of falling film. Methods to improve the performance of falling film absorption.
【學位授予單位】：東南大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TK124;TB657

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本文編號：2177001